Hope you're still accepting these this late on a Friday.... It's hard to keep it down to only five since anything outside the Kirby/Sinnott Marvel matrix was underappreciated by me when I was a kid. But off the top of my head:

Listing was tough for me ... even as a child I dug Sekowsky, Ditko, Heck, Alcala, Elias, Robbins, etc. - any of the guys usually thrown in these "can't stand 'em" list. In retrospect, it was probably a comparison thing i.e. "Trimpe ain't no Kirby" or "Moldoff ain't no Sprang." However, the use-to-like-or-tolerate-but-can't-stand-'em-now list would go far longer than this superflorous paragraph.

And yes, it is a mystery to me as well why I have come to appreciate Don Heck's artistry while still not caring for Tuska's work, despite what seem to be a lot of similarities in their work to my eyes...

The list of artists that I didn't appreciate when I was young and still don't appreciate now would be a far, far longer list that would include Jack Kirby.

*****

Robert Ullman

Mike Mignola (the first time I saw his work was in Alpha Flight #29, which had previously been illustrated by John Byrne. My thirteen-year-old brain exclaimed "They traded John Byrne for this guy? He doesn't even draw in Heather Hudson's eyeballs!")
Bill Sienkiewicz (Same situation as before, 'cept he took over New Mutants from, I believe, Tom Palmer)
Ramona Fradon (I always preferred the Aquaman reprints that Aparo drew to Fradon' s...now I realize it's an "everybody wins!" type of thing)
Gene Colan
Russ Heath

Trying to think back to my earliest comic reading days here, which I assume is what you're asking about. I could list underground guys like Crumb, et al, who just freaked me out as an older kid. But judging by this list, I sure disliked the Caniff school and offshoots quite a bit as a kid. It didn't help that guys like Robbins and Elias were not in their prime and doing material ill-suited to their styles at the time I came upon their work (including The Invaders/Captain America and The Human Fly, of all things). Anyway, there you go.